One type of semiconductor die having a raised topology is known in the art as a "bumped" die. A bumped die includes bond pads formed with a bump of solderable or bondable material. Bumped dice are used for flip chip bonding and for tape automated bonding (TAB). With these bonding systems, the die is mounted face down to a supporting substrate, such as tape or a printed circuit board, and then bonded to a corresponding connection point, such as a substrate pad.
The bumps not only secure the die to the substrate membrane but also establish electrical communication between the bond pads on the die and the corresponding connection points on the substrate. For flip chip bonding, the bond between the bump and a substrate pad can be formed by reflow soldering. For TAB bonding, the bond between the bump and conductors formed on flexible tape can be formed using thermocompression bonding. Bumps are also used for "chip on glass" and "chip on flex" systems that utilize a z-axis conductive material and a variety of substrate materials.
Typically, the bumps are formed as balls of material with a flat tip and a diameter of from 50 .mu.m to 100 .mu.m. The bumps can be formed using plating or evaporation of metals through holes in a metal mask. For example, it is common practice to form the bumps out of a solderable material such as gold or a tin/lead alloy. Gold or tin/lead bumps can be formed on a bond pad of a die using a plating, evaporation, sputtering, or ball bonding process. These plating processes for gold can also include forming a barrier layer out of a material such as titanium/tungsten. Evaporation and sputtering processes can also be used to form solderable lead/tin bumps for a bonding process developed by IBM and known as C4.
One disadvantage of metal mask fabrication processes for bumps is that capital expenditure for process equipment is high. The high capital expenditure can be justified if the bumps are being used on a high volume product. However, for some products, such as field emission displays, the capital costs associated with forming custom bumped semiconductor dice can be prohibitive.
Another disadvantage of bumps formed of gold is that the bond pads on a die are typically formed of aluminum. Because the bond pads and bumps are formed of dissimilar materials, galvanic reactions, poor adhesion, thermal mismatch and brittle intermetallic compounds can result. One widely known reliability problem occurs at the gold-aluminum interface wherein intermetallic compounds such as AuAl.sub.2 (purple plague) are formed. Another problem, known as "Kirkendahl voiding", can occur at elevated temperatures, wherein aluminum diffuses rapidly into the AuAl.sub.2 phase and leaves voids at the interface.
These problems can occur not only during the bump-substrate bonding process but also during subsequent heat cycling of electronic products that utilize bumped semiconductor dice. For example, field emission displays are constructed using heat cycles that can approach 400.degree. C. These high temperatures can cause the formation of intermetallics between the gold bumps and aluminum pads, resulting in purple plaque and Kirdendahl voiding.
Because pads on semiconductor dice are typically formed of aluminum, it would be desirable to be able to also form the bumps for bond pads out of aluminum. This would simplify the bump structure and would increase the reliability of the interconnection between the bump and pad. However, the properties of aluminum make aluminum bump formation difficult.
In view of the foregoing, it is an object of the present invention to provide an improved method for forming bumps on the pads of semiconductor dice using aluminum wire. It is a further object of the present invention to provide an improved method for forming bumps on semiconductor dice using aluminum wire and conventional modified ball bonding apparatus. It is a still further object of the present invention to provide an improved method for forming field emission displays, and other electronic devices, with semiconductor dice having ball bumps formed using aluminum wire.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.